Hart was inspired to draw cavemen (and many other creatures) through the chance suggestion of one of his coworkers at General Electric, and took to the idea "because they are a combination of simplicity and the origin of ideas." The name for the strip "may have been suggested by my wife, Bobby," Johnny recalls.[8]

Hart describes the title character as similar to himself, playing the "patsy." The other major characters — Peter, Wiley, Clumsy Carp, Curls, and Thor — were patterned after friends and co-workers. The animal characters include dinosaurs, ants and an anteater, clams, a snake, a turtle and bird duo, and an apteryx (presented in the strip as being the sole surviving specimen, and hence self-aware of its being doomed to extinction).

Peter: A yellow haired, self-styled genius and the world's first philosophical failure, founder of the "Prehistoric Pessimists Society" and the "Truth Pedestal," and the discoverer of oil. Peter is patterned after Hart's friend, Peter Reuter; the two had been co-workers at General Electric.

Thor: a self-proclaimed ladies' man; inventor of the wheel and the comb. Thor was patterned after another of Hart's friends from GE, Thornton Kinney.

The Fat Broad: a bossy cavewoman who enjoys clobbering snakes. A reluctant arbiter of congeniality with an unswerving devotion to the domination of men.

The Cute Chick: a sex object in a world that had not yet discovered objectivity.

Wiley: a peg-legged, superstitious, unshaven, woman-fearing, water-hating poet and coach of the local baseball and football teams, not to mention the first bartender. Wiley was patterned after Hart's brother-in-law, Wiley Baxter, who lost his leg in World War II.

Clumsy Carp: a nerdy, bespectacled ichthyologist and perpetual klutz, clumsy enough to trip over a shadow, yet with some unusual skills, such as his ability to make and stack "water balls" (similar to snowballs). Clumsy Carp was patterned after Hart's childhood friend, Jack Caprio.

Curls: a master of sarcastic wit. Curls was patterned after Hart's friend from high school, Richard (Curly) Boland.

Grog: pure Id, a caveman's caveman; a primitive, semi-evolved wild man with a one-word vocabulary and enough strength to knock the sun out of the sky using a golf ball.

The Guru: an unnamed, bearded wise man living like a hermit atop a mountain, whence he dispenses wisdom and sarcasm.

John the Turtle and the Dookie Bird: this prehistoric odd couple are inseparable friends, especially when making their annual trek south for the winter. The Dookie Bird rides on John's back when they travel.

The Snake: the put-upon, mortal enemy of the Fat Broad (and her club).

The Eatanter: eats ants with a sticky, elastic tongue and a ZOT! sound. Hart actually drew something of a hybrid—with the long ears of an aardvark and the bushy tail of a giant anteater. (This character was the inspiration for Peter the Anteater, the University of California, Irvineteam mascot. Also served as the inspiration for the mascot of the now disbanded US Navy fighter squadron VF-114 the "Aardvarks".)[9]

Maude: an ant, a nagging wife with a smart-alec son (Johnny) and a quarrelsome, straying husband.

Jake: ant husband of Maude, who is always threatening to run off with Shirley.

Queen Ida: the queen ant, an unfeeling and abusive dictator. (Queen Ida is based on Hart's wife Bobby, whose given name is Ida. She's featured every year on her birthday, December 3.)

The Dinosaur: big but not too bright—a sort of sauropod with spinal plates like a stegosaurus. Sometimes called Gronk, which is the only sound he makes (although he can talk fluently in recent strips).

The Clams: talking clams with legs, among other appendages. (Clams are also the preferred unit of currency in B.C.)

The Apteryx (kiwi): a "wingless bird with hairy feathers," as he invariably introduces himself.

The Turkey: makes his yearly appearance at Thanksgiving time, eluding the mighty hunters.

Oynque: the turkey's porcine partner in crime, rarely seen without his trademark mud puddle.

Although the strip seldom expands its human cast outside of the established group of characters, there are a few exceptions. It can be assumed that there are other groups or tribes of humans for Wiley's sports teams to compete against, for example, but these are never actually seen. There have been a few exceptions to this, however, with a few additional human characters seen from time to time, even if only once.

Anno Domini, or A.D., introduced during a weeks-long journey by Peter to discover the new world, which he successfully accomplished. His name is arguably a riff on B.C.'s name. He dresses as a caveman very much like the rest of the characters, but has a thick mustache and a stereotypical Italian accent, assuming a bit of a take on Christopher Columbus. He befriends Peter in the "new world."

Conahonty, a Native American Indian, who also appears in the "new world" storyline, and befriends Peter. He is a friend of A.D.'s, and speaks rather stereotypical broken English. He dresses more like a somewhat stereotyped Indian than a caveman, and at one point even specifically states that he is an American Indian. He and A.D. were not frequently seen after Peter returned from his epic journey. The two are the most oft-appearing non-regular human characters in the history of the strip other than the Guru, due to the strip's tight focus on its core cast of humans. His name is a spoof of the name Pocahontas.

Peter's Pen Pal, An unseen person whom Peter corresponds with by tossing letters written on wooden slabs into the ocean and receiving answers at a later time.

The characters live, for the most part, in caves, in what appears to be a barren, mountainous desert by an unidentified sea. Background detail is often limited to a simple horizon line broken up by the occasional silhouettes of a stray volcano or cloud. "Retail stores", "shop counters", and "businesses" are symbolized by a single boulder, labeled (for instance): "Wheel Repair", "Advice Column", "Psychiatrist", etc. The February 5, 2012, strip gives a nearby location of N 53° 24' 17" W 6° 12' 3", which is in present-day Dublin, Ireland.

Originally, the strip was set firmly in prehistoric times, with the characters clearly living in an era untouched by modernity. Typical plotlines, for example, include B.C.'s friend, Thor (inventor of the wheel and the comb), trying to discover a use for the wheel. Thor was also seen making calendars out of stone every December. Other characters attempt to harness fire or to discover an unexplored territory, like Peter trying to find the "new world" by crossing the ocean on a raft. Animals, like the dinosaur, think such thoughts as, "There's one consolation to becoming extinct—I'll go down in history as the first one to go down in history." Grog arrived in early 1966,[11] emerging from a miniature glacier which melted to reveal what Wiley called "Prehistoric man!"

B.C., like Hart's Wizard of Id, is a period burlesque with a deliberately broad, unliteral time frame. As time went on the strip began to mine humor from having the characters make explicit references to modern-day current events, inventions, and celebrities. Increasingly familiar visual devices, like the makeshift "telephone" built into a tree trunk, also started to blur the comic's supposed prehistoric setting and make it rife with intentional anachronisms. One of the comic's early out-of-context jokes, from June 22, 1967, was this one:

Peter: "I used to think sun revolved around the earth."

B.C.: "What does it revolve around?"

Peter: "The United States!"

Another early example: Near Christmas time, the apteryx, dressed as Santa Claus, modified his usual spiel: "Hi there, I'm an Apterclaus, a wingless toymonger with batteries not included!" The Washington Post columnist and comics critic Gene Weingarten suggested[12] that B.C. is actually set not in the past but in a dystopic, post-apocalyptic future.

B.C. follows a gag-a-day format, featuring (mostly) unrelated jokes from day to day, plus a color Sunday page. Occasionally it will run an extended sequence on a given theme over a week or two. It also follows the convention of Sunday strips with a short, setup/payoff joke in the first two panels, followed by an extended gag. The principal cast is small and varied, with each character imbued with a developed personality. "The art style, like that of Charles Schulz's Peanuts, masks sophisticated minimalism with a casually scratchy veneer," according to comics historian Don Markstein.[13]

Dry humor, prose, verse, slapstick, irony, shameless puns and wordplay, and comedic devices such as Wiley's Dictionary (where common words are defined humorously with a twist, see Daffynition) make for some of the mix of material in B.C. Example: "Rock (verb): To cause something or someone to swing or sway, principally by hitting them with it!"—from an early 1967 strip. Or: "Cantaloupe (noun): What the father of the bride asks after seeing the wedding estimate!"

There are running gags relating to the main cast and to a variety of secondary, continuing characters. One such periodic recurring gag has Peter communicating with an unseen pen-pal on the other side of the ocean, writing a message on a slab of rock that he floats off into the horizon. It is invariably returned the same way, with a sarcastic reply written on the reverse side. These segments use silent or "pantomime" panels (indicating that time has elapsed; night falls and dawn rises) between the set-up and the delayed punch line—typical of Hart's idiosyncratic use of "timing" in B.C.

Late in the run of the strip, and following a renewal of Hart's religious faith in 1984, B.C. increasingly incorporated religious, social, and political commentary, continuing until Hart's death in 2007. References to Christianity, anachronistic given the strip's supposed setting and the implications of its title, became increasingly frequent during Hart's later years. In interviews, Hart referred to his strip as a "ministry" intended to mix religious themes with secular humor.[14] Though other strips such as The Family Circus and Peanuts have included Christian themes, B.C. strips were pulled from comics pages on several occasions due to editorial perception of religious favoritism or overt proselytizing. Easter strips in 1996 and 2001, for example, prompted editorial reaction from a handful of U.S. newspapers, chiefly the Los Angeles Times and written and oral responses from Jewish and Muslim groups. The American Jewish Committee termed the Easter 2001 strip, which depicted the last words of Jesus Christ and a menorah transforming into a cross, "religiously offensive" and "shameful."[15] A 2003 strip depicting a character using an outhouse with a crescent symbol on the front, slamming the door shut, and declaring, "Is it just me, or does it stink in here?" was interpreted by some as carrying an anti-Islam message. Hart responded to the controversy, saying "This comic was in no way intended to be a message against Islam — subliminal or otherwise.... It would be contradictory to my own faith as a Christian to insult other people’s beliefs."[16][17] The Los Angeles Times consequently relegated strips which its editorial staff deemed objectionable to the religion pages, instead of the regular comics pages.[18]

The B.C. daily strip from December 7, 2006 attracted criticism for defining infamy as "a word seldom used after Toyota sales topped 2 million." The day was the 65th anniversary of the Japanese military's attack on Pearl Harbor, and the punchline of the strip refers to Franklin D. Roosevelt's "Infamy Speech" which requested from Congress a declaration of war against Japan. The day's strip was pulled from at least one newspaper, the San Antonio Express-News. The paper's managing editor said the comic was "a regressive and insensitive statement about one of the worst days in American history."

On July 21, 2009, the strip presented a gag that involved the supposed suggestion of animal abuse. John Hart Studios received many angry responses from readers and issued an apology on their website.[19]

The characters appeared in animated commercials for the U.S. Federal agency ACTION in the 1970s and for Monroe shocks in the late 1980s. They were also licensed by Arby's restaurants in 1981, which issued a collector set of 6 B.C. cartoon character drinking glasses. In the last half of the 1960s, the BC characters were used in commercials for Marathon gasoline.

The Broome County parks department[21] features Gronk the dinosaur as their mascot, and Thor riding a wheel graces every Broome County Transit bus. In the past, Hart has also left his mark on the logos of the Broome Dusters and B.C. Icemen hockey teams.